1. Technical Field
The present invention relates generally to the field of rotary-wing aircraft and more particularly to the field of power transmissions for rotary-wing aircraft.
2. Description of Related Art
The driveshaft, or mast, of a rotary-wing aircraft is typically outfitted with a means for carrying rotor blades in a manner such that the rotor blades are capable of selectively producing lift when the mast is rotated. Of the rich variety of means known for transferring torque to a mast, a common configuration is to associate the mast with a single large bull gear (either by connecting the bull gear to the mast with splines or by forming the bull gear integrally with the mast) and driving the bull gear with a transfer gear. While this configuration is common, it remains fraught with a number of unfortunate design limitations.
More specifically, while it is often desirable to drive a mast with high torque, the size and/or weight of a bull gear capable of transmitting the high torque over a sufficiently long period of time may prove to be a significant limiting factor. As with other gears, a design concern with bull gears is gear tooth bending, which is a common mode of failure in bull gears. As a bull gear is driven by a transfer gear, the teeth of the transfer gear transmit force from the faces of the teeth of the transfer gear to the faces of the teeth of the bull gear. If there is only one transfer gear driving the bull gear, each tooth of the bull gear is said to undergo one bending cycle per revolution of the bull gear. Since the bending force endured by each individual tooth of the bull gear is a determining factor of when gear tooth failure of the bull gear teeth will occur, it is desirable to keep the bending force as low as possible. Prevention of gear tooth failure is imperative because failure of a bull gear may lead to a catastrophic rotary-wing aircraft failure.
In attempts to reduce the bending force applied to the teeth of bull gears, some have driven the bull gear by splitting the torque and delivering the torque to the bull gear with multiple transfer gears. For example, by driving a bull gear with two identical transfer gears, the nominal bending force applied to the teeth of the bull gear would typically be reduced by one-half as compared to the force when using only one transfer gear. This is a seemingly elegant solution, but in practice, a number of problems occur in implementing the solution. First, it is important to note that each discrete drive path in the above described configuration has an inherent natural frequency at which the transmission must be prevented from oscillating. Also, as is easily recognized by those of ordinary skill in evaluating system dynamics, other modes of vibration and composite natural frequencies related to the interaction of the two independent drive paths exist. Oscillation at each of the other composite natural frequencies must also be avoided during operation of the transmission.
Further, previous attempts at driving a bull gear with multiple inputs were troubled by less than optimal distribution of torque between the multiple drive paths and the force fights which occur between the two drive paths when a phase difference or lead/lag situation occurs between the drive paths. For the two drive-path configuration as described above, any inconsistencies in the meshing of the first transfer gear to the bull gear (whether caused by debris, inconsistent tooth wear, etc.) would transmit force back through the first drive path to the torque splitting device and ultimately through the second drive path to the second transfer gear. The second transfer gear then transmits an atypical force to the teeth of the bull gear. Since the atypical forces transferred to the teeth of the bull gear may easily be of very high magnitude (at least higher than the force normally transferred to the teeth of the bull gear in a single transfer gear configuration), this force fighting is incompatible with the goal of reducing the forces transmitted to the teeth of the bull gear.
While the torque transfer mechanisms described above represent significant developments in transmitting torque to a bull gear, considerable shortcomings remain.